Language: incomplete interface implementation compile-time fatal (Zend zend_inheritance.c parity) #3386

Category

language

Problem

When a class implements an interface but omits a required method, Zend fails at compile time:

Fatal error: Class C contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (I::f)

This compiler accepts the class and runs it — verified May 2026:

interface I { public function f(): int; }
class C implements I {}
echo "ok\n"; // prints ok on bin/vm.php

php-src reference

• php/php-src Zend/zend_inheritance.c — zend_do_implement_interface, abstract method count
• php/php-src Zend/zend_compile.c — zend_verify_abstract_class

Repro (failure today)

<?php
interface I { public function f(): int; }
class C implements I {}
echo "ok\n";

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php repro.php; echo vm:$?'
# prints ok, vm:0 (wrong)

php repro.php; echo zend:$?
# zend:255 — Fatal error: Class C contains 1 abstract method…

Variant: wrong signature

interface I { public function f(): int; }
class C implements I { public function f(): string { return 'x'; } }

Zend: return type compatibility error at compile time (coordinate with #3323 variance).

Scope

Done when

• Missing method repro → compile-time fatal (message matches Zend subset)
• Class with all methods implemented → compiles and runs
• Multiple interfaces — report first missing method or aggregate count like Zend
• ./script/ci-fast.sh --filter interface_missing green

Links

• Language: Traits, abstract classes, and interfaces #144 umbrella — claim this issue for the compile-time check only
• Compiler: Class inheritance (extends) and interfaces #101 extends / instanceof interface
• Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492